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I'm hacking together a RF-controlled garage door opener. I'm tapping the "receiving" end in to the push button wires.

When the two wires touch, it toggles the garage door motor.

I'm trying to basically "close" this circuit through a 5V Trinket Pro with...

pinMode(A0,OUTPUT); in setup and digitalWrite(A0,LOW); in the loop.

Using the multimeter, it would seem that the wires for the push button are normally closed (I think that's correct) as I get a 15v read on the multimeter when I check the current.

When I touch the wires, the current drops to 0.

My understanding was that the digitalWrite(A0,LOW); would basically "complete" the circuit, but it doesn't seem to do anything.

So, my question is: Given I've only got 5v running through the Trinket Pro, is that not enough to toggle the garage door circuit given it's putting out 15v in it's NC state?

If that's the case, is there some other way around that without needing to push 15v of power?

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The Trinket really doesn't want to see 15v across one of its I/O pins. You'll do better using a solid-state (SSR) or mechanical relay to close the 15v motor-start circuit, one that can be activated by the 5v output of the Arduino pin.

Update:
A relay has 4 specs that you need to consider up front:

  • The coil (or switching input of a SSR) voltage, and
  • The coil (or switching input) current;

These are what the Arduino or an add-on switching circuit will have to drive. And:

  • The contact max voltage, to limit arcing when they open
  • the contact max current, to limit them heating,

Or, in the case of an SSR, to prevent letting out the Magic Smoke that makes them work.

If you use a relay you'll need a driver circuit between the Arduino pin and the relay, with a snubber - a diode that grounds the reverse voltage "kickback" from the relay coil when you open its circuit - also to protect the Magic Smoke in your Arduino.

You're hacking into your remote "clicker", not the garage-door tractor itself, right? If that's the case, just about anything with a 5v coil 15v+ contacts will do. The remote button only has to switch a tiny transmitter on/off. If you do need to hack into the tractor, you'll want to find out what's behind its receiver terminal. It's probably not the motor but another relay, but....

A ready source of such a relay is your/another garage door remote receiver. It has a relay in it that is already doing that job.

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  • Is there a certain type of relay I'm looking for in regards to the 5v/15v pairing?
    – Shpigford
    Jun 25, 2016 at 1:22

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